CHARLOTTESVILLE, Va. — A Virginia police chief said Monday that he “absolutely has regrets” about violence that erupted over the weekend when dozens of white nationalists, neo-Nazis and Ku Klux Klan members clashed with counterprotesters.

As the world watched pandemonium in Charlottesville unfold live on television Saturday, officers seemed to stand on the sidelines as fists flew, bats swung and objects soared through the air.

“We were hoping for a peaceful demonstration,” Chief Al Thomas said at a news conference. “Gradually the crowd size increased along with aggressiveness and hostility of the attendees towards one another.”

After police ordered everyone out of a small park where the rally was being held, protesters took to the streets. A man plowed his car into a group of counterprotesters, killing a woman and injuring 19 others.

On Monday, Virginia Gov. Terry McAuliffe said he directed his administration to conduct an “extensive review” of how police prepare and respond to rallies. The city’s former police chief and law enforcement experts were critical of the way police responded, saying the ostensibly hands-off approach seemed to allow the violent fracas to grow.

Thomas said his officers were spread thin and had to make quick adjustments to their strategies when white nationalists began swarming the park and violence erupted.

Charlottesville officers called for backup from state police, who helped clear out the park. But police appeared to have no clear plan of what to do after that, as protesters flooded out of the park onto the streets.

The Charlottesville police department had responded to 250 calls Saturday and emergency workers treated 36 injured people, including several who were hurt in the car attack, Thomas said.

“Absolutely I have regrets,” Thomas said. “It was a tragic, tragic weekend.”

Former Charlottesville Police chief Timothy Longo said he was “shocked” that police didn’t block the street where a car plowed into a group of protesters Saturday, killing 32-year-old Heather Heyer and injuring several others.

“I’m shocked in disbelief that there was traffic anywhere near that level of pedestrian activity. That street is typically shut down on weekends for events that occur on the mall,” Longo, who retired from the department last year, told CNN.

Joseph Giacalone, a professor at John Jay College of Criminal Justice and former New York City police sergeant, said there should have been a stronger police presence and officers should have created a buffer zone between the two groups. Longo said he also would have expected police to have created buffers.

“When you have a group on one side and another group protesting against them, you have to put yourself in the middle of them,” Giacalone said. “As uncomfortable as it is to be a police officer out there in between them, that’s the only thing to do.”

Those on both sides of the protests were also critical of how police responded to the violence.

“Their entire mission seemed to be to just stand there,” said Matthew Heimbach, a leader of the white nationalist Traditionalist Worker Party.

Andrew Mayton, a union organizer and researcher who traveled from Baltimore to protest the white supremacists for the second time this summer, said officers were “nowhere to be found.”

Giacalone, who helped plan the NYPD’s response to Occupy Wall Street, said it appeared police in Charlottesville chose not to engage with the protesters and surmised officials may not have wanted to appear as militarized as officers who used war-fighting gear to confront protesters in Ferguson, Missouri, after the fatal shooting of Michael Brown.

“I think there’s this disengagement theory that’s been pushed forward,” he said. “You’re dealing with radicals and a disengagement strategy is not going to work with them.”

Thomas denied any accusation that his officers had been told not to make arrests or step in when fights broke out.

City officials had tried to move the rally to a larger park about a mile from downtown Charlottesville, but their request was blocked by a federal judge after the American Civil Liberties Union brought a lawsuit challenging the proposal. Virginia’s governor, a Democrat, said he believed the violence could have been quelled if the rally had been moved to the larger park. The ACLU said its lawsuit challenged the city to act constitutionally.

“Had that been up there, we wouldn’t have had the issues. We wouldn’t have had the car terrorism, and let me be very clear: It was terrorism,” McAuliffe said.